If bypassing the lanyard doesn't solve this issue, you will need to follow the path of the dreaded purple wire and inspect, clean and apply dielectric grease to all connectors this passes through. A little corrosion at a connector has the potential to open the circuit in the same way as the tether switch can.

Haha, so I take it this is a semi-common problem for older boats? That is another concern of mine (corrosion). I bought the boat in TX and the original owner had it in GA and their is lots of rust on it...